It is common in the manufacture of marine propulsion devices, such as outboard motors and stern drives, to construct the device with a primary exhaust discharge and a secondary exhaust discharge. The primary exhaust discharge generally directs exhaust into the low pressure area below the water and behind the propeller. This can be accomplished either by directing the exhaust through an outlet on the underside of the anti-ventilation plate or through a hollow propeller hub. Noxious exhaust gases and noise are generally retained in the water.
The secondary exhaust discharge is generally located above the top of the water level when the marine propulsion device is mounted on an associated boat. The secondary exhaust discharge is generally designed to reduce the back pressure of the exhaust gas in the marine propulsion device when the engine is idling or is running at low speed.
Accordingly, the secondary exhaust discharge is commonly referred to as an idle relief passage. Since the idle relief passage bypasses the normal underwater exhaust discharge system, it is designed to minimize the exhaust noise transmitted through it by being designed with a series of baffles. However, in many marine propulsion devices, the idle relief passage allows some noise, along with various noxious exhaust gases, smoke, and in some cases water or steam, to emanate from this outlet under most operating conditions, especially at slow speed.
When a boat is operated for an extended length of time at slow speed a considerable amount of exhaust gas may emanate from the secondary exhaust gas discharge. In certain conditions, such as when the wind is blowing from the stern, these gases can be blown into the boat and cause discomfort to the operator and passengers.
U.S. Pat. No. 3,552,121 shows a marine propulsion device including a vacuum valve in the exhaust for relieving negative pressure created during engine start.
U.S. Pat. No. 4,601,666 discloses an outboard motor with a selectively operable valve in the exhaust passage which diverts all of the exhaust to either the primary under water exhaust discharge port or the secondary above water exhaust discharge port in order to divert exhaust to the secondary exhaust discharge port during slow speed trolling operation in order not to disturb the fish. The diverter is associated with the engine throttle.
U.S. Pat. No. 5,106,330 discloses an outboard motor with a secondary exhaust outlet that automatically shuts off upon high powerhead speed.